Field of the Invention
The present invention relates to a radiation generating device that emits X-rays in a wavelength range of 1 pm to 10 nm, which is applicable for medical equipment, a non-destructive inspection apparatus, and the like.
In particular, aspects of the present invention relate to a transmission-type X-ray target including a target layer and a diamond substrate that supports the target layer. Moreover, aspects of the present invention relate to a radiation generating tube including the transmission-type X-ray target, to a radiation generating device including the radiation generating tube, and a radiographic apparatus including the radiation generating device.
Description of the Related Art
In recent years, home care systems have been established and the levels of care provided by emergency medical services have been increased. Accordingly, there is an increasing demand for small and lightweight medical modality apparatuses that are portable. In order to respond to such needs, various medical modality apparatuses have been developed as analysis and diagnostic techniques in the medical field have progressed. Exiting radiographic apparatuses including radiation generating devices are large. Therefore, existing radiographic apparatuses are mainly floor-mounted types used in hospitals and medical examination facilities. To date, such floor-mounted medical modality apparatuses have been used by setting operating periods and non-operating periods, which include maintenance periods.
It is desirable that such radiographic apparatuses including radiation generating devices have high durability and be designed for easy maintenance, so that the operating rate of the apparatus can be increased and the apparatuses can be used as medical modality apparatuses suitable for home-care and emergency medical services in a case of a natural disaster or an accident.
A main factor that determines the durability of a radiation generating device is the durability of a target, which serves as a generation source of radiation, such as X-rays. Typically, a target includes stacked layers. It is known that, in order to increase the durability of a target, it is important to maintain the cohesion of the layers of the target for a long time.
For a radiation generating device that generates radiation by irradiating a target with an electron beam, the radiation generation efficiency of the target is 1% or less. Therefore, almost all of the energy provided to the target is converted into heat. If the heat generated by the target is not sufficiently dissipated to the outside, problems may arise in that the material of the target may degenerate or a problem related to the cohesion, such as separation of a layer due to a stress generated between layers, may occur.
It is known that the radiation generation efficiency can be increased by using a transmission-type X-ray target including a target layer that is a thin film including a heavy metal and a substrate that transmits radiation and that supports the target layer. PCT Japanese Translation Patent Publication No. 2009-545840 describes a rotary-anode-type transmission-type X-ray target having a radiation generation efficiency that his 1.5 times higher than those of existing rotary-anode-type reflection-type targets.
It is known that heat can be efficiently dissipated from the target to the outside by using diamond as the material of a substrate for supporting a target layer of a transmission-type X-ray target. PCT Japanese Translation Patent Publication No. 2003-505845 describes a technology for increasing heat-dissipation performance and realizing a microfocus X-ray device by using diamond as the material of a substrate for supporting a target layer made of tungsten.
Diamond has exceptional characteristics due to strong sp3 bonds and highly regular crystal structure. Among the characteristics of diamond, high heat resistance (melting point of 1600° C. or higher), high thermal conductivity (600-2000 W/m/K), high radiation transmissivity (attributable to being a light element of atomic number 6) are particularly suitable for use as a substrate for supporting a transmission-type X-ray target. In a transmission-type X-ray target according to the present invention, a substrate for supporting a target layer is made of diamond.
However, diamond has low compatibility with a target metal because wettability of a molten metal on diamond is low and the coefficient of linear expansion differs between a solid metal and diamond. Therefore, it may be necessary to improve cohesion between a target layer and diamond substrate in order to improve the reliability of a transmission-type X-ray target.
PCT Japanese Translation Patent Publication No. 2003-505845 describes a transmission-type X-ray target that includes a diamond substrate and that has a stacked structure in which an intermediate layer is disposed between a diamond substrate and a target layer. The intermediate layer serves as a cohesion enhancing layer, although the material of the intermediate layer is not described.
Japanese Patent Laid-Open No. 2002-298772 describes a problem of a radiation generating tube including a transmission-type X-ray target. The problem is that a target layer may become separated from a diamond substrate due to the difference in the coefficient of linear expansion and heat generated when the radiation generating tube is operating. Japanese Patent Laid-Open No. 2002-298772 describes a technology for suppressing separation of the target layer by forming the target layer so as to be convex toward the diamond substrate. Japanese Patent Laid-Open No. 2002-298772 also describes a technology for increasing the reliability of electrical connection between the target layer and an anode member by disposing the target layer so as to extend beyond the periphery of the diamond substrate and overlap the anode member.
However, even with the transmission-type X-ray targets described in PCT Japanese Translation Patent Publication No. 2003-505845 and the Japanese Patent Laid-Open No. 2002-298772, in which cohesion of the target layer and the diamond substrate is improved, variation in radiation output may occur.